Formation of leuco compounds



Patented Jan. 7, 1936 UNITED STATES PATENT OFFICE FORMATION or LEUCOCOMPOUNDS Louis S. Bake, Pennsgrove, N. J., assignor to E. I. du Pont deNemours & Company, Wilmington, DeL, a corporation of Delaware NoDrawing. Application February 27, 1930, Serial No. 431,984

17 Claims.

This invention relates to a method of reduction of organic compounds andmore particularly to the reduction of vat colors to their leucocompounds. It especially contemplates the reduction of indigo to itsphenol, or leuco compound, known commercially as indigo white.

In present commercial use there are two satisfactory general. methodsfor the reduction of indigo to indigo white. In the first method the'The zinc hydrosulfite is in turn preliminarily produced by theinteraction of sodium bisulfite or gaseous sulfur dioxide with metalliczinc as follows:

The second general method involves the reducing action of zinc-andsodium bisulfite upon the indigo probably according to the followingequa- I This method has the disadvantages that the sodium bisulfitenecessary to carry out the reac- 35 tion is lost as a by-product in itstransformation to the normal sulfite and that the sodium bisulfite mustbe preliminarily prepared, ordinarily by the reaction of sodiumhydroxide and sulfur dioxide as follows:

40 N aOH+S OHNaHS 03.

It is an object of this invention to produce an 7 5 improved process forthe reduction of indigo to its leuco compound. Further objects are toproduce an improved quality of indigo white and in general to overcomethe disadvantages of the prior art. Further objects will appear herein.-

55 after.

These objects are accomplished by the present invention whereby thereduction is brought about by the use of zinc hydrosulfite. Zinchydrosulfite may be added to the water suspension of indigo blue and asuccessful reduction 5 secured, but it is preferred to form the zinchydrosulfite in the vat. Accordingly, the zinc hydrosulfite is formeddirectly in the indigo water suspension by'adding to it metallic zincdust and passing in gaseous sulfur dioxide, the 10 reaction taking placeas follows:

, The zinc hydrosulfite thus formed reacts immediately with the indigoto form the phenol and zinc bisulfite according to the followingreaction:

The zinc bisulfite which is formed is insoluble in water, as is theindigo phenol and in order to separate them it is convenient to convertthe zinc into a soluble compound. This can be readily done by treatmentof the vat mixture with an acid to cause formation of zinc salts thatare soluble, the indigophenol being insoluble in acid solution.,Satisfactory acids are hydrochloric and sulfuric. The reaction takingplace on the addition of the acid is as follows:

If an amount of zinc hydrosulfite theoretically necessary to reduce allthe indigo were used a large quantity of. sulfur dioxide would be lostas is obvious from the above equation (Equation 3). This general processis therefore modified to avoid this loss.

According to this modification the indigo is made into a watersuspension which is neutral or slightly acid and an amount of zincslightly more than necessary to form the theoretical amount of zinchydrosulfite necessary for the reduction ofv all the indigo is added.Because of reactions of minor importance in the vat some zinc may belost,- therefore a slight excess is added. Thereafter, gaseous sulfurdioxide in a smaller amount than is necessary to combine with all thezinc to form zinc hydrosulfite is passed into the suspension.

The solution is then heated from to C. and approximately 0.95 of thetheoretical amount of sulfuric acid necessary to convert all the zincbisulfite (which would be formed by the zinc present) into zinc sulfateis added either continuously or in separate portions over 55 a period of15 to 30 minutes or longer, at which time reduction is complete. Asubstantial excess of acid at the end of the reduction tends to causethe formation of ,indigo blue, therefore slightly less than themolecular proportion of acid is used. The acid is not all added at onetime quickly as it would probably react with the metallic zinc directly.The phenol is then separated and washed in any desired manner.

The theoretical explanation of the modified process is probably asfollows. The small relative amount of sulfur dioxide added in thebeginning reacts with its equivalent of metallic zinc forming anequivalent amount of zinc hydrosulfite which reduces a correspondingmolecular amount of indigo, forming zinc bisulfite which is reacted upon(Equation 3) by the acid present liberating an amount of sulfur dioxideequal to the amount added originally, minus of course, any small amountthat may have been oxidized by oxygen which may have been present. Thisregenerated sulfur dioxide reacts in exactly the same manner as thatoriginally added, by combining with its equivalent of the remaining zincto form more zinc hydrosulfite. Thus, the above cycle continues untilthe reduction is complete and the zinc is in solution as zinc sulfate.

The above reactions (1), (2), and (3), may be assumed empirically toproceed simultaneously or in sequence so that the reaction goes tocompletion without further attention.

While in the majority of cases the above reduction proceeds smoothly inneutral indigo suspension, cases have been found when a poor grade ofzinc dust has been used that an activator was necessary to start thereaction, which, when once started, went smoothly. to completion. Suchactivators are hydrochloric and sulfuric acids, ammonium chloride andzinc chloride. These compounds work very efiiciently when present in anamount necessary to give the original indigo and water suspension adistinct acid reaction to litmus paper.

The invention will be readily understood from a consideration of thefollowing examples:

Example I Indigo C16H10N2O2 I l H H Two hundred pounds of 'neutral 20%indigo paste were placed in the reducing tub, 250 pounds of water addedand the suspension stirred thoroughly while 4 pounds of sulfur dioxidein the gaseous state were added. Fourteen pounds of zinc dust availablezinc), as a slurry inwater, was then added and the temperature raised toapproximately 70 C. Eighteen pounds of sulfuric acid as a 30% solutionwere added continuously over a period of 30 minutes. Stirring wascontinued until the reduction was complete, when the indigo phenol wasfiltered and washed free from zinc sulfate.

Example II Two hundred pounds of neutral 20% indigo paste were placed inthe reduction tub, 250 pounds of water added and the agitator started.Zinc chloride solution was added until the suspension was distinctlyacid to litmus paper, and then 4 pounds of gaseous sulfur dioxide wereadded. Fourteen pounds of zinc dust (95% available zinc), as a slurry inwater, was then added and the temperature raised to approximately 70 C.Eighteen pounds of sulfuric acid as a 30% solution were addedcontinuously over a period of 30 minutes. Stirring was continued untilthe reduction was complete, when the indigo phenol was filtered andwashed free from zinc sulfate.

Example III Two hundred pounds of neutral 20% indigo paste were placedin the reduction tub, 250 pounds of water added and the agitatorstarted. Ammonium chloride solution was added until the suspension wasdistinctly acid to litmus paper, and then 4 pounds of gaseous sulfurdioxide added. Fourteen pounds of zinc dust (95% available zinc), as aslurry in water, was then added and the temperature raised toapproximately 70 C. Eighteen pounds of sulfuric acid as a 30% solutionwere added continuously over a period of 30 minutes. Stirring wascontinued until the reduction was complete, when the indigo phenol wasfiltered and washed free from zinc sulfate.

Example IV Two hundred pounds of neutral 20% indigo paste were placed inthe reduction tub, 250 pounds of water added and the agitator started.Hydrochloric acid solution was added until the suspension was distinctlyacid to litmus paper, and then 4 pounds of gaseous sulfur dioxide added.Fourteen pounds of zinc dust (95% available zinc), as a slurry in water,was then added and the temperature raised to approximately 70 C.Eighteen pounds of sulfuric acid as a 30% solution were addedcontinuously over a period of 30 minutes. Stirring was continued untilthe reduction was complete, when the indigo phenol was filtered andwashed free from zinc sulfate.

Example V Two hundred pounds of neutral 20% indigo paste were placed inthe reduction tub, 250 pounds of water added and the agitator started.Sulfuric acid solution was added until the suspension was distinctlyacid to litmus paper, and then 4 pounds of gaseous sulfur dioxide added.Fourteen pounds of zinc dust (95% available zinc), as a slurry in water,was then added and the temperature raised to approximately 70 C.Eighteen pounds of sulfuric acid as a 30% solution were addedcontinuously over a period of 30 minutes. Stirring was continued untilthe reduction was complete, when the indigo phenol was filtered andwashed free from zinc sulfate.

Ninety-six pounds of the vat color, Ponsol yellow G (neutral 22% paste),Colour Index #1118, were placed in the reducing tub, 200 pounds of wateradded, the agitation started, and 3 pounds of gaseous sulfur dioxideadded. Four pounds of zinc dust were then added as a slurry in water andthe temperature raised to 35-50 C. Four pounds of sulfuric acid wereadded continuously over a period of 15-30 minutes, when the reductionwas complete. Then the insoluble leuco compound was filtered off,suspended in water and dissolved in caustic, after which it was readyfor use.

Example VII Pyranthrone C30H14O2 soluble leuco compound was thenfiltered off,

dissolved in dilute caustic. It was then ready for use.

In the above examples it does not matter whether the sulfur dioxide orthe zinc dust is added first, nor does it matter if the solution isheated before either are added, except that of course sulfur dioxide ismuch less soluble in the indigo water suspension at the highertemperature, provided the zinc has not been added. The amount of watermay be either increased or decreased without lowering the efficiency ofthe reduction, except in a mechanical manner. The amount of sulfurdioxide may be either increased or decreased without injury to thereaction. It is clearly evident that increasing the amount of sulfurdioxide over that required to complete the reduction would result onlyin a waste of gas, and very disagreeable operating conditions due to,excess sulfur dioxide in the building. The amount of sulfur dioxide asgiven in the above examples may be reduced to such an extent that thereis present over and above that oxidized by any free oxygen present inthe solution, only a slight excess at the end of the reduction. Ifnonoxidizing conditions are maintained, as for example by means of anatmosphere of inert gas such as nitrogen'or carbon dioxide, and/or theoperation carried out in a closed vessel to prevent loss of sulfurdioxide during the reduction process, only a few ounces or less ofsulfur dioxide are necessary. This results from the continuousregeneration feature as previously explained in detail.

While certain theories of operation have been set out for the newprocess, it is to be understood that they are given merely toaid inunderstanding the invention and applicant does not desire to be limitedthereby, but only by the appended claims.

As many apparent and widely different em- 5 bodiments of this inventionmay be made without departing from the spirit thereof, it is to beunderstood that I do not limit myself to the foregoing examples ordescription except as indicated in the following claims. 10

'I claim:

1. In the reduction of indigo to leuco indigo, the steps of adding to anindigo suspension sufficient zinc to form with sulfur dioxide sufficientzinc hydrosulfite to reduce the indigo 15 present and adding to theindigo suspension less than the amount of sulfur dioxide required tomolecularly combine with the zinc, the amount of sulfur dioxide addedbeing sufiicient to form zinc hydrosulfite in the solution andthereafter 20 adding sulfuric acid to the reaction mixture.

2. In the reduction of indigo to leuco indigo, the steps of adding to anindigo suspension suflicient zinc to form with sulfur dioxide sufficientzinc hydrosulfite to reduce the indigo 5 present and adding to theindigo suspension less than the amount of sulfur dioxide required tomolecularly combine with the zinc, the amount of sulfur dioxide addedbeing sufficient to form zinc hydrosulfite in the solution andthereafter 30 adding a nonoxidizing mineral acid to the reactionmixture.

3. The process of treating indigo which comprises forming a watersuspension of the indigo,

making the suspension acid with a non-oxidiz- 35 ing inorganicsubstance, adding enough zinc to form sufficient zinc hydrosulfite toreduce the indigo present to its leuco compound, and adding sulfurdioxide.

4. The process of treating indigo which com- 0 prises forming a watersuspension of the indigo,

'making the suspension acid in reaction with zinc chloride, addingenough zinc to form sufficient zinc hydrosulfite to reduce the indigopresent to its 55 leuco compound, adding about 10% of the amount ofsulfur dioxide required to molecularly combine with the zinc, the amountof' sulfur dioxide added being sufficient to form zinc hydrosulfite inthe solution, to form zinc hydro- 5 sulfite, allowing the sulfur dioxideto react with the zinc to form zinc hydrosulfite and allowing the zinchydrosulfite to reduce the indigo.

6. The process of treating indigo which comprises forming a watersuspension of the indigo, as adding less than the amount of sulfurdioxide required to molecularly combine with zinc, the amount of sulfurdioxide added being sufiicient to form zinc hydrosulfite in thesolution, to form sufficient zinc hydrosulfite to reduce the indigopresent, adding enough zinc to form sufficient zinc hydrosulfite toreduce the indigo present to its leuco compound, allowing the sulfurdioxide to react with the zinc to form zinc hydrosulflte and allowingthe zinc hydrosulfite to reduce the indigo, and adding sufiicienthydrochloric acid to reduce all zinc bisulfite formed to zinc chloride.

'7. The process of reducing vat dyes to their leuco compounds whichcomprises, under nonoxidizing conditions passing into a water suspensionof the dye, a small quantity of sulfur dioxide thereafter adding to thesuspension approximately the theoretical amount of zinc required to formsuificient zinc hydrosulfite to reduce the dye present, and then slowlyadding to the mixture approximately the theoretical quantity ofnon-oxidizing mineral acid required to convert any zinc bisulfite formedtozinc sulfate and sulfur dioxide and finally separating the leucocompound.

8. The process of reducing vat dyes to their leuco compounds whichcomprises, in a nitrogen atmosphere passing into a water suspension ofthe dye a small quantity of sulfur dioxide thereafter adding to thesuspension slightly more than the theoretical amount of zinc required toform sufficient zinc hydrosulfite to reduce the dye present, and thenslowly adding to the mixture slightly less than the theoretical quantityof non-oxidizing mineral acid required to convert any zinc bisulfiteformed to a soluble zinc salt and sulfur dioxide and finally separatingthe leuco compound.

9.The process of reducing vat dyes to their leuco compounds whichcomprises, in a carbon dioxide atmosphere passing into a watersuspension of the dye a small quantity of sulfur dioxide thereafteradding to the suspension approximately the theoretical amount of zincrequired to form sufficient zinc hydrosulfite to reduce the dye present,and then slowly adding to the mixture approximately the theoreticalquantity of non-oxidizing mineral acid required to convert any zincbisulfite formed to zinc sulfate and sulfur dioxide and finallyseparating the leuco compound.

10. A process of reducing a reducible organic compound which comprisesmixing the organic compound with metallic zinc and an amount of sulfurdioxide in water insufficient to interact with all the zinc present, andduring the reaction adding an amount of sulfuric acid to form andmaintain the presence of zinc hydrosulfite from the zinc bisulfiteformed.

11. The process of reducing indigo blue to indigo white which comprisesforming a suspension from 200 parts of neutral 20% paste and 250 partsof water, then during agitation passing in 4 parts of gaseous sulfurdioxide, thereafter adding a slurry of zinc dust and water containing 14parts of zinc of 95% availability and adjusting the temperature to 0.,then adding 18 parts of 30% sulfuric acid continuously over a period of30 minutes, stirring until the reaction is complete and separating theindigo white.

12. The process which comprises reducing vat dyes in an open vessel byadding slightly more sulfur dioxide to the dye than will be oxidizedduring the reaction, the amount of sulful dioxide added being sufiicientto form zinc hydrosulfite in the solution, thereafter adding sufficientfinely divided zinc to form sufficient zinc hydrosulfite to reduce thedye to its leuco form, heating the resultant mixture to a temperaturebelow which the sulfur dioxide will be expelled, then passing into thereaction mass continuously at a rate insufiicient to react with the zincpresent, a quantity of sulfuric acid sufficient to release sulfurdioxide by reaction with any zinc bisulfite present.

13. The process which comprises reducing vat dyes in an open vessel byadding slightly more sulfur dioxide to the dye than will be oxidizedduring the reaction, the amount of sulfur dioxide added being sufficientto form zinc hydrosulfite in the solution, thereafter adding suflicientfinely divided zinc to form sufficient zinc hydrosulfite to reduce thedye to its leuco form, heating the resultant mixture to a temperaturebelow which the sulfur dioxide will be expelled, then passing into thereaction mass intermittently at a rate insufficient to react with thezinc present, a quantity of sulfuric acid sufficient to release sulfurdioxide by reaction with any zinc bisulfite present.

14. A process of reducing a vat color to its leuco which comprisesmixing the vat color with metallic zinc and an amount of sulfur dioxidein water insufficient tointeract with all the zinc present, and duringthe reaction continuously adding an amount of sulfuric acid to form andmaintain the presence of zinc hydrosulfite from the zinc bisulfiteformed.

15. A process of reducing a vat color to its leuco which comprisesmixing the vat color with metallic zinc and an amount of sulfur dioxidein water insufficient to interact with all the zinc present, and duringthe reaction intermittently adding an amount of sulfuric acid to formand maintain the presence of zinc hydrosulfite from the zinc bisulfiteformed.

16. A process of reducing a vat color to its leuco which comprisesmixing the vat color with metallic zinc and an amount of sulfur dioxidein water insufficient to interact with all the zinc present, and duringthe reaction adding an amount of a non-oxidizing mineral acid to formand maintain the presence of zinc hydrosulfite from the zinc bisulfiteformed.

17. The process of treating indigo, which comprises forming a watersuspension of the indigo,

adding to the said suspension a substance from

